A 50-year-old woman presents with intermittent fever, a recurrent generalized headache, malaise, and fatigability for two weeks. She has had a painless reddened lump on her right arm for the same duration. Her medical, surgical, and drug histories are unremarkable. Three weeks ago, she returned from a one-month vacation in Costa Rica. She was in good health during the entire vacation. During this time, she engaged in multiple outdoor activities, including hiking, rock climbing, swimming in rivers, and snorkeling. There is no history of unsafe sexual contact or intravenous drug abuse. She is unmarried, has no children, and lives alone.